The method for coating a substrate with silver nanoparticles includes reducing a silver nitrate solution with an ethanol extract of the traditional Indian medicinal plant (Curcuma Longa L.), a naturally abundant antioxidant, to form a final solution, and contacting the final solution with the substrate to provide the silver nanoparticle coating. Formation of the silver nanoparticle coating on the substrate can be determined when a mirror in the final solution is observed. The thickness of the coating layer can be less than 125 nm. The coated substrates can be highly conductive.